The present invention relates to a drive circuit for driving a group of driven elements, an optical print head as an LED (Light emitting diode) head including the drive circuit; and an image forming apparatus such as an electro-photography printer including the optical print head. More specifically, the present invention relates to a drive circuit for selectively driving per cycle the group of the driven elements such as an array of LEDs of an image forming apparatus using LEDs as a light source, an array of heating resistor members in a thermal printer, and an array of display elements in a display device.
In a conventional image forming apparatus such as an electro-photography printer using an electro-photography process, an optical print head is formed of a plurality of light emitting elements (for example, LEDs) arranged linearly, so that each of the LEDs emits light to form an image.
In the LED print head, a drive circuit is disposed for driving each of the LEDs. The drive circuit is formed of a drive IC (Integrated Circuit) chip such as a field effect transistor (referred to as an FET). More specifically, a plurality of drive IC chips is arranged linearly on a print circuit board in a pattern corresponding to an arrangement of the LEDs, and the drive IC chips are fixed with a thermally setting resin capable of being thermally cured (refer to Patent Reference).
Patent Reference: Japanese Patent Publication No. 2007-329295
In the conventional LED print head described above, a circuit board unit thereof is formed of the drive IC chips fixed to the print circuit board cured under a high temperature environment. In general, the print circuit board has a thermal expansion coefficient larger than that of the drive IC chips. Accordingly, when the circuit board unit is moved from the high temperature environment to a room temperature environment, a large compressive stress is created in the drive IC chips in an arrangement direction thereof. The compressive stress is applied to a drive transistor in the drive IC chip, thereby varying a drive current thereof. When the drive current is fluctuated, the LED driven with the drive IC chip emits light with fluctuated luminescence. As a result, the electro-photography printer using the conventional LED print head forms an image with a fluctuated density, thereby lowering print quality.
In view of the problems described above, an object of the present invention is to provide a drive circuit capable of solving the problems of the conventional drive circuit.
Further objects and advantages of the invention will be apparent from the following description of the invention.